Distribution transformers are relatively large electrical transformers, typically between 10 Kva and 50 Kva, commonly used to reduce voltage from 2000-25,000 volts to 110/220 volts for residential and commercial use. Because of the large amount of electricity handled by distribution transformers, efficiency is of prime concern in their design.
There are several methods for winding electrically conductive coils for distribution transformers. In one method, a series of coils is wound separately. A strip of core material is then wound through the center of the coils. See, for example, U.S. Pat. No. 2,191,393 to Humphreys, U.S. Pat. No. 4,381,600 to Mas, and U.S. Pat. No. 4,741,484 to Curtis, the disclosures of which are incorporated by reference. This method is not now used to any substantial extent. Another method, extensively used for commercial production of distribution transformers, proceeds by winding the coil onto a winding form or mandrel apart from the magnetic core. The winding form or mandrel defines the size and shape of the opening in the coil. The core is subsequently inserted through the opening in the coil. See, for example, U.S. Pat. No. 3,340,489 to Bastis, the disclosure of which is incorporated by reference.
There are disadvantages to both of these prior art methods. First, there must be working clearance between the core and the coil. Second, the coil opening must be sized to accommodate the maximum core size, allowing for normal manufacturing tolerances. Third, the first method requires that the coil have a maximum radius of the core cross section, plus a working clearance for winding the coil onto the bobbin. Fourth, the second method requires that the magnetic circuit be cut and opened for insertion into the coil, and then reclosed.
Enlargement of the coil opening to accommodate different size cores requires that the electrical conductors be longer to obtain the same number of turns. Inefficiency, or load loss, of the transformer is directly affected by the conductor length. Therefore, the required spacing between the electrical conductors and the core results in less efficient transformers.
Another problem, referred to above, relates to variation in the dimension of the magnetic core. Magnetic cores are necessarily layered structures with certain space between the laminations. For certain types of materials, such as amorphous magnetic core material available from Allied Signal, Morristown, New Jersey as Metglas TCA, the manufacturer stipulates a range of void space from up to 30%. However, for a fixed cross section of a magnetic core, having a specified level of magnetic induction, the gross cross section of the core would vary by +11% to -11%.
Two cores can have the same magnetic cross section; however, the core which is more tightly spaced, so that there is less void space between the layers, will have a higher gross density. The core having a lower gross density will necessarily have a longer mean magnetic path and will have greater weight of energized magnetic core material. This results in increased core loss proportional to the increase in weight. To reduce the core loss, more magnetic material may be added to reduce the magnetic flux density for an offsetting value. This, however, compounds the variation in the gross cross section of the material +18% to -13%.
Specific core loss of magnetic core materials also varies within a certain percentage from the mean. This variation can be offset by decreasing the magnetic flux density or by increasing the amount of material in the core. This, of course, adds further variation to the gross cross section of the core.
It has been found that a conventionally wound distribution transformer using a spool of Metglas TCA magnetic core material requires that, using conventional distribution transformer manufacturing techniques, the inner perimeter of the coil be designed about 10% larger than a nominal size to accommodate variations in the size of the core. As discussed above, doing so lowers the efficiency and thus increases the cost of use of the distribution transformer.